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6 Photodegradation of Pesticides and Photocatalysis

H2O2



Fe



261



2+



OH



OH + OH



Fe(OH)2+





solar light



Scheme 6.6 Photocatalytic cycle of iron in the photo-Fenton process



As shown in the photocatalytic cycle, the continuous production of hydroxyl

radicals as well as the presence of hydrogen peroxide leads to the oxidation of the

organic pollutants and also of the generated by-products. This phenomenon permits, in many cases, the total mineralisation of the solution [89].



6.9 Conclusions

In conclusion, current research on the degradation of organic pollutants has been

focused on the combination of chemical (AOP’s) processes and biological treatment and these have been tested on biorecalcitrant compounds such pesticides and

chlorophenols [76, 90–92]. The results have shown that such AOP’s have the

ability to degrade the pollutants and produce by-products with high bio-degradation efficiency. Furthermore, by using the combination of biological and chemical

treatments, in addition to improving the removal efficiencies, there is also a

marked reduction of the overall costs, thus overcoming one of the main economic

hindrance to practical exploitation.

Powdered TiO2 is widely used in suspension (slurry) with excellent performance, but it requires filtration. Immobilisation of TiO2 on solid supports such as

glass, polymers and ceramics removes the need for filtration to separate the catalyst from treated water, making the process less expensive.

Solar photocatalytic degradation of water contaminants with titania and photoFenton catalysts has been carried out on a pilot-plant scale at the solar photochemical facilities of the Plataforma Solar de Almeria (PSA) in Spain, and show

how solar photocatalysis is likely to become important within the next few decades

in wastewater treatment and development of new AOP technologies [93, 94].

The testing and development of reactors to maximise the optical efficiencies are

also being carried out at PSA. Unfortunately, at present there are no general rules

for the photocatalysed degradation of several pollutants, and preliminary research

is always required to optimise the best conditions.



262



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Chapter 7



Solar Energy Conversion

Luis G. Arnaut, Monica Barroso and Carlos Serpa



Abstract Photochemical conversion of solar photons is one of the most promising

and sought after solutions to the current global energy problem. It combines the

advantages of an abundant and widespread source of energy, the Sun, and Earthabundant and environmentally benign materials, to produce other usable forms of

energy such as electricity and fuels, without the negative impact of CO2 or other

greenhouse gas release into the atmosphere. Dye-sensitised solar cells (DSSC) and

organic bulk heterojunction (BHJ) solar cells are two examples of such systems,

allowing the conversion of visible sunlight into electricity by inorganic or organic

semiconductor materials, which are inexpensive and easy to process on a large

scale. Photocatalytic (PC) and photoelectrochemical (PEC) water splitting systems

offer a solution to the problem of diffuse and intermittent sunlight irradiation, by

storing the energy of solar photons in the form of clean energy vectors such as H2.

This chapter presents an overview of the technologies based on photochemical

solar energy conversion and storage.



L. G. Arnaut (&) Á M. Barroso Á C. Serpa

Department of Chemistry, University of Coimbra,

3004-535 Coimbra, Portugal

e-mail: lgarnaut@ci.uc.pt

M. Barroso

e-mail: m.barroso@imperial.ac.uk

C. Serpa

e-mail: serpasoa@ci.uc.pt

M. Barroso

Department of Chemistry, Imperial College London, London, SW7 2AZUK



R. C. Evans et al. (eds.), Applied Photochemistry,

DOI: 10.1007/978-90-481-3830-2_7,

Ó Springer Science+Business Media Dordrecht 2013



267



268



L. G. Arnaut et al.



7.1 Introduction

Every day the Earth is supplied with an immense amount of energy by the Sun.

Approximately 4.3 9 1020 J of photon energy hit the surface of our planet each

hour, which is enough to cover our yearly energy needs [1–3]. This is our largest

and most widely available renewable energy source and it is therefore surprising

that solar-based energies contribute only a minute portion of the world’s energy

demand. On the other hand, fossil fuels, which supply *85 % of the current

energy consumption [2], are finite and unevenly distributed beneath the Earth’s

surface. In addition, burning these leads to the release of harmful gases into the

atmosphere. While the imminent depletion of fossil fuel and coal reserves is

subject to some controversy, it is widely accepted that alternative energy sources,

which avoid or compensate the release of anthropogenic CO2 and other greenhouse gases, are urgently required.

Current solar energy technology and research are focused on the conversion of

solar photons into electricity (photovoltaics), chemical energy (solar fuels) and

heat (solar thermal). In this chapter we will explore the first two approaches.

Converting the energy from the Sun into electricity is a challenging but

important task. However, large quantities of electrical energy cannot be stored

easily and efficiently, and the intermittent nature of solar emission means that

electric energy can only be obtained during daylight hours. This is not compatible

with the rhythm of modern society. Solar fuel production is an attractive solution

to this problem due to its potential to generate low-carbon energy carriers, which

can store the energy contained in solar photons and effectively replace fossil fuels.

The paradigm of the clean solar fuel technology is the light-driven splitting of

water into hydrogen and oxygen gases, employing abundant and environmentally

friendly semiconducting photoelectrode materials. Alternatively, solar water

splitting can be combined with photochemical or electrochemical CO2 reduction to

generate carbon-based products that can be used either as fuels or feedstock for

several reactions with industrial value.

The scientific and technical challenges posed by these strategies are manifold

and range from the fundamental limitations related to inefficient light harvesting,

charge separation and recombination, to applied aspects associated with device

architecture and function. Although remarkable progress has been achieved in

understanding the scientific basis of the processes involved in solar energy conversion, low overall efficiencies and high production costs remain major obstacles

to the widespread use of solar energy technologies. Therefore, it is likely that the

capturing, converting and storing solar energy will remain one of the most

important topics of research in the coming years.



7 Solar Energy Conversion



269



7.2 Dye-Sensitised Solar Cells

7.2.1 Fundamentals

Efficient dye-sensitised solar cells (DSSC) are based on the synergy between

sensitiser dyes capable of absorbing sunlight, and convenient wide band gap

nanocrystalline semiconductors. The existence of functional groups on the dye

molecules capable of efficiently binding to the semiconductor surface (the

anchoring groups) and the high surface area of nanocrystalline mesoporous

materials, enable the binding of sufficient dye molecules to absorb most of the

incident light, with the inherent potential of maximising the light harvested.

The basic photophysical properties of a dye molecule are maintained upon

immobilisation on a semiconductor surface, but the interaction with the semiconductor may open new reactive routes and/or change the rate of particular

photochemical processes. An example of the importance of these routes is the fact

that some polypyridyl complexes, intrinsically photolabile in solution, become

photostable when bound to the semiconductor titanium dioxide, and actually

constitute the class of dyes that has enabled some of the most efficient cells

constructed to date [4, 5].

In a dye-sensitised solar cell the fundamental new reactive route for the immobilised dye is an electron transfer reaction. The excited state generated by light

absorption is a molecular excited state. Then, charge separation that follows light

absorption requires an electron transfer from the molecular electron donor (the dye

molecule) to the semiconductor (electron acceptor). Proper control of the kinetics

of the this first electron transfer step is a primary condition for having an efficient

dye-sensitised solar cell. The electron transfer leads to the formation of a chargedseparated state constituted by the dye oxidised state (the dye cation or hole) and

the electron on the semiconductor. One of the major origins of inefficiency in solar

cells is the charge recombination of this pair. Dye-sensitised solar cells partly

overcome the problem of charge recombination by completely separating the

processes of light absorption, achieved by the sensitiser dye, and charge transport

and collection. After the formation of the charge-separated state, electron transport

occurs through the network of sintered semiconductor nanoparticles, whilst a

suitable redox couple enables the regeneration of the sensitiser.

The schematic diagram in Fig. 7.1a, the energy level diagram of Fig. 7.1b and

the chemical Eqs. 7.1–7.5 are parallel but complementary ways of describing a

dye-sensitised solar cell. The equations are written for a generic sensitiser dye (S)

but illustrate the most commonly

À

Á used semiconductor (titanium dioxide, TiO2) and

À

. The schematic diagrams can be followed through

=I

the typical redox pair IÀ

3

the description of the basic steps occurring in a typical DSSC: upon light

*

absorption by the dye (Eq. 7.1) the excited

À À dye

Á (S ) transfers an electron into the

conduction band of the semiconductor ecb , resulting in an oxidised dye and a

reduced semiconductor (this process is often termed electron injection, Eq. 7.2).



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L. G. Arnaut et al.



Fig. 7.1 a Schematic diagram illustrating the architecture and main electronic transition steps

occurring in a typical DSSC. White circles represent semiconductor particles and the black circles

the dye molecules; charge flow is represented by arrows. b Energy level diagram illustrating the

relative energy position of DSSC main components: semiconductor, sensitiser dye and

electrolyte. Electron injection from hot states (kinj*) and relaxed states (kinj) of the excited

dye, and charge recombination (krec) between the electron in the semiconductor conduction band

and the dye cation, are represented by arrows



The injection process is in kinetic competition with the relaxation processes of the

dye in its excited state (Eq. 7.3). Efficient electron transport within the semiconductor allows the collection of electrons on the back contact of the electrode,

typically a thin layer of fluorine doped tin oxide (F:SnO2, or FTO) or indium doped

tin oxide (In:SnO2, or ITO) deposited on a transparent substrate, like glass. This

electrode is known as photoanode because it promotes the photo-oxidation of the

sensitiser and moves electrons to the external circuit. The redox electrolyte in

contact with the sensitised semiconductor reduces the oxidised dye, regenerating

the sensitiser, and transports the resulting positive charge to the counter electrode

(Eq. 7.4). This electrode (the cathode, as it collects positive charges) is usually a

conducting glass covered with a transparent platinum or carbon thin coating. An

external circuit connecting this sandwich-like structure allows the transport of the

collected electrons from the anode to the cathode. These electrons will promote the

reduction of the redox mediator (Eq. 7.6), closing the circuit. Well-known sources

of efficiency loss in DSSC involve transfer of electrons across the semiconductor/

electrolyte interface, either to oxidised dye molecules (charge recombination,

Eq. 7.5) or to the oxidised component of the redox couple (charge interception,

Eq. 7.7).

Excitation:

TiO2 jjS þ hm ! TiO2 jjSÃ :



ð7:1Þ



þ

TiO2 jjSÃ ! eÀ

cb À TiO2 jjS :



ð7:2Þ



Injection:



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